A solid laser is a laser using a solid for a laser medium. A solid laser amplification device is a device for increasing (amplifying) the output of a laser light by passing the laser light through a laser medium. For example, in the case of a slab type solid laser amplification device, a laser light entering a laser medium advances in a zigzag manner in the laser medium while being reflected by both the opposite surfaces of the laser medium, whereby amplification is performed.
The laser medium is heated by the laser light passing through the inside thereof, and therefore, cooling is required. In the related art, direct cooling is used for the cooling of the laser medium. In the direct cooling, the cooling is performed, for example, by pressurizing the interior of a cooling chamber provided so as to cover the surface of the laser medium and injecting liquid nitrogen toward the surface of the laser medium in the cooling chamber.